1. Field of the Invention
The present invention relates to an etchant for copper or copper alloys useful in the manufacture of printed circuit boards and the like.
2. Description of the Background Art
Microetching is performed in the production of printed circuit boards to remove contamination and oxides from the surface of copper and to obtain an active copper surface. Microetching is an operation of slightly etching a surface, usually less than 5 xcexcm in depth. Microetching is performed as a pretreatment for lamination or coating of resins such as an etching resist, solder resist, insulating layer, a pretreatment for electroless plating or electroplating, a pretreatment for soldering, and the like. In particular, when resins are laminated or coated, microetching is performed to produce a roughened surface exhibiting improved adhesion with the resins.
Conventionally, persulfate-based microetching compositions, sulfuric acid-hydrogen peroxide-type microetching compositions, and the like are used as microetching compositions.
However, conventional persulfate-based microetcing compositions have drawbacks such as a low etching rate and fluctuation in the etching rate due to unstable properties of etching solutions caused by decomposition of persulfate compounds. Another problem is related to metals other than copper, such as a tin lead alloy (solder), nickel, and gold, used in printed circuit boards. When microetching such a printed circuit board, the tin lead alloy or nickel may also be etched or discolored.
Sulfuric acid-hydrogen peroxide type microetching compositions also have such problems as instability of etching solutions due to decomposition of hydrogen peroxide which results in fluctuation in the etching rate, discoloration of metals other than copper, and the like.
An alkaline etchant containing copper ammonium complex ion as a major component is used in a circuit patterning step in the process of photoetching. This alkaline etchant also exhibits instability in the liquid composition, namely the etching rate tends to fluctuate. An additional problem with this etchant is a strong ammonia odor which impairs working environmental conditions.
Japanese Patent Publication No. 55-15512 discloses a neutral etchant containing a copper (II) ion source such as copper (II) chloride and a complexing agent such as monoethanolamine, and an alkaline etchant prepared by further adding an ammonium salt such as ammonium chloride. However, this neutral etchant exhibits a low etching rate and the alkaline etchant imparts an ammonia odor.
Japanese Patent Application Laid-open No. 6-17266 discloses a surface treating composition containing a copper (II) ion source such as copper (II) chloride and an alkanolamine, which the applicant claims is useful in removing gray or black films formed on a lead frame surface etched using an aqueous solution of iron (II) chloride or the like. However, this surface treating composition cannot be used as an etchant because of a low copper etch rate.
Japanese Patent Application Laid-open No. 7-292483 discloses a surface treating composition which is an aqueous solution containing a copper (II) complex of an azole compound, an organic acid, and a halide ion, and optionally alkanolamine. However, the amount of alkanolamine in this surface treating composition must not be in excess of the equivalent amount of the organic acid. If alkanolamine in excess of the amount of the organic acid is added, the copper etching rate is retarded.
Therefore, an object of the present invention is to provide an etchant in which the above problems in conventional etchants have been overcome, namely, the present invention provides an etchant free from problems such as instability of the liquid composition and unpleasant odor, and exhibiting a high etching rate.
Another object of the present invention is to provide an etchant exhibiting only very slight corrosion even if a small amount of residue is left on the surface, and capable of producing a roughened surface when used for microetching.
The inventors of the present invention have conducted extensive studies and have been successful in solving the above-described problems in the prior art by using the following etchant.
Specifically, the above object is solved in the present invention by an etchant for copper or copper alloys comprising 5-50wt % (hereinafter simply indicated by xe2x80x9c%xe2x80x9d) of an alkanolamine, a copper ion source in the amount of 0.2-10% as copper, a halide ion source in the amount of 0.005-10% as halogen, 0.1-30% of an aliphatic carboxylic acid, and the balance water, wherein the molar ratio of the alkanolamine to one mol of the aliphatic carboxylic acid is two or more.
In a preferred embodiment of the above etchant, the copper ion source is a copper (II) ion source, the amount of the halide ion source is 0.005-5wt % as a halogen, the amount of the aliphatic carboxylic acid is 2-30 wt %, and the etchant has a pH in the range of 7.5 to 11.5.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.
The alkanolamine is a component acting as a complexing agent to dissolve copper. A low molecular weight alkanolamine having a molecular weight of 500 or less is preferable, because such a low molecular weight alkanolamine produces a low viscosity etching solution which ensures uniform etching and is suitable for etching to form fine circuit patterns.
As specific examples of such an alkanolamine, monoethanolamine and its derivatives such as N-methylethanolamine, N-ethylethanolamine, N-butylethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N,N-dibutylethanolamine, 2-(2-hydroxy)ethoxyethanolamine; diethanolamine and its derivatives such as N-methyldiethanolamine and N-butyldiethanolamine; triethanolamine, propanolamine, iso-propanolamine, and hydroxy ethylpiperazine, as well as derivatives of these may be given.
The concentration of alkanolamines is 5-50%, and preferably 10-30%. If the concentration of alkanolamines is less than 5%, the etching rate becomes low; if more than 50%, the viscosity of the etchant increases. When a copper complex of an alkanolamine, which is hereinafter discussed, is used as a copper ion source, an alkanolamine in the amount of 5% or more is added in addition to the alkanolamine forming the copper complex. In this instance, the upper limit of 50% for alkanolamines in the etchant must be observed.
In addition, the ratio of the alkanolamine to one mol of the aliphatic carboxylic acid, which is discussed later, is two mols or more. If this ratio is less than two mols, the etching rate is retarded.
The copper ion source is a component which forms a complex with the above-described alkanolamine and acts as an oxidant of copper. As the copper ion source, the above-mentioned copper complexes of alkanolamine, copper hydroxide, copper complexes of aliphatic carboxylic acid which are discussed later, copper halides such as copper chloride and copper bromide, copper carbonate, copper sulfate, copper oxide, and the like can be given. The copper ion sources may be either the compounds which generate the copper (I) ion or the compounds which generate the copper (II) ion. When the concentration of halide ion described later is less than 5%, the compounds generating the copper (II) ion are preferred due to their capability of dissolving copper in a stable manner.
The concentration of the copper ion sources is 0.2-10% as copper, and preferably 1-6% as copper. If the concentration is less than 0.2%, the etching rate becomes low. The etching rate also becomes low when the concentration is more than 10%. When a copper halide is used as the copper ion source, the copper halide should be added in such an amount that the concentration of the halide ion does not exceed 10%.
The halide ion sources increase copper solubility in the etchant, copper dissolution stability, and the etching rate. In addition, the halide ion sources exhibit an action of roughening copper surfaces. As specific examples of the halide ion sources, inorganic acids such as hydrochloric acid, hydrobromic acid, and hydroiodic acid, and inorganic salts such as copper chloride, copper bromide, iron chloride, sodium chloride, sodium iodide, ammonium chloride, and the like can be given. Of these, the compounds which generate a bromide ion are preferred when the etchant of the present invention is used for microetching those surfaces of copper or copper alloys which must be roughened due to their high capability of roughening such surfaces.
The concentration of the halide ion sources is 0.005-10% as halide ion. If this concentration is less than 0.005%, the etching rate is retarded. On the other hand, an effect proportionate to the amount of addition cannot be expected if more than 10% of the halide ion sources is added. In addition, when used for microetching those surfaces of copper or copper alloy swhich are roughened, such excessive addition may interfere with the capability of the etchant of roughening such surfaces. Furthermore, the above concentration is preferably 5% or less in the case where residues of the etchant are apt to remain on the treated surfaces. If more than 5%, the residues of the etchant remaining corrode the treated surfaces.
An aliphatic carboxylic acid is added to the etchant of the present invention to increase dissolving stability of copper and to adjust the pH.
Given as specific examples of aliphatic carboxylic acids are aliphatic saturated monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, and caproic acid; aliphatic unsaturated monocarboxylic acids such as acrylic acid, crotonic acid, and isocrotonic acid; aliphatic saturated dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, and pimelic acid; aliphatic unsaturated dicarboxylic acids such as maleic acid; aliphatic oxycarboxylic acids such as glycolic acid, lactic acid, malic acid, and citric acid; aliphatic carboxylic acids with substituents such as xcex2-chloropropionic acid, hydroxypivalic acid, and levulinic acid; and derivatives of these aliphatic carboxylic acids.
Of these, aliphatic monocarboxylic acids are preferable because of their properties of not interfering with the copper oxidizing activity of alkanolamine complexes and a low viscosity of the resulting etching solution.
The concentration of the aliphatic carboxylic acids is 0.1-30%, and preferably 0.1-15%. If the concentration of the aliphatic carboxylic acids is less than 0.1%, the effect of the addition is insufficient. The addition of more than 30% of the aliphatic carboxylic acids, on the other hand, decreases the pH, which results in a low etching rate. When the concentration of halide ion is less than 5%, the concentration of aliphatic carboxylic acids in the range of 2-30%, and particularly 7-15%, is preferable in view of the etching rate. When a copper complex of an aliphatic carboxylic acid is used as the copper ion source, the concentration of aliphatic carboxylic acids, including the aliphatic carboxylic acid which forms the complex, is from 0.1 to 30%.
In addition to the above components, a rust preventive agent such as benzotriazole, a surfactant for ensuring uniform etching such as polyalkylene glycol and polyalkylene glycol derivatives, and the like may be added to the etchant of the present invention. In addition, a salt of the above-mentioned aliphatic carboxylic acid, a salt of the above-mentioned alkanolamine, a salt of inorganic acid, and the like may be added to improve the properties of the solution and to stabilize the surface configuration when the etchant of the present invention is used for roughening surfaces.
Although the etchant of the present invention maybe acidic, neutral, or alkaline according to the kind and concentration of the alkanolamine and aliphatic carboxylic acid, it is desirable to control the pH in the range of 7.5-11.5, and more particularly 8.2-10.8, to maintain a high etching rate when the halogen concentration is less than 5%.
The etchant of the present invention can be easily prepared by dissolving the above-described components in water. Ion-exchanged water is preferably used as the water for this purpose. There is no specific limitation to the method of applying the etchant of the present invention. For example, a method of spraying the etchant to copper or a copper alloy, a method of immersing the copper or copper alloy in the etchant, and the like are given. The temperature of the etchant is preferably from 10 to 50xc2x0 C., and the treating time is preferably from 10 to 120 seconds, and particularly preferably 10 to 60 seconds in the case of microetching.
When used for microetching copper or a copper alloy, the etchant of the present invention can not only remove stains and oxides from the surface thereby producing an active surface, but also can produce an irregular surface which exhibits superior adhesion to resins. Therefore, the etchant of the present invention is useful as a pretreatment for lamination or coating an etching resist or a solder resist, a pretreatment for lamination or coating an interlayer insulation resin, a pretreatment for electroless plating or electroplating, a pretreatment for forming a rust preventive film such as a preflux for printed circuit boards, and the like.
In addition, because the resulting surface has excellent solderability, the etchant of the present invention is also useful in a pretreatment for a hot air leveler process and soldering of electronic parts. For the solder used for the above-mentioned soldering, in addition to a tin-lead eutectic solder, lead-free solders such as tin-copper-bismuth-based solder, tin-silver-copper-based solder, tin-silver-bismuth-based solder, and tin-zinc-based solder can be used. The etchant of the present invention can also be preferably used for surface finishing of lead frames, and the like. Furthermore, because the etchant of the present invention does not corrode a tin-lead alloy, nickel, or gold, the etchant is useful for treating printed wiring boards and semiconductor parts in which these metals are present together with copper.
Moreover, because the etchant of the present invention may etch the surface opposed to spray nozzles faster than the other surfaces using a spraying method, the etchant can be preferably used for high aspect ratio etching in a photoetching process for forming circuit patterns on printed circuit boards. Control of the etching rate is also easy. Although tin, tin lead alloys, nickel/gold laminated layers, and the like can be given as etching resists in addition to thermosetting resins and photosensitive resins, tin, tin lead alloys, nickel, and gold which are not corroded by the etchant of the present invention can also be used as etching resists.
A semi-additive method which is one method for fabricating printed circuit boards involves a process of forming a circuit pattern which comprises steps of forming a base conductive layer of electroless plating copper film, a copper sputtering film, or a copper foil, forming a plating resist thereon, forming a copper electroplating film on an insulating material surface, removing the resist, then removing the base conductive layer on which the copper electroplating film has not been formed by etching. The process has an additional step of roughening the surface of the circuit pattern by microetching to improve adhesion with a solder resist or an interlayer insulation resin. This microetching step can be omitted if the etchant of the present invention is used for removing the base conductive layer because the etchant can roughen the circuit surface simultaneously when removing the base conductive layer. Therefore, a problem of making a narrow circuit by microetching in the case of a fine circuit can be overcome. In addition, when the base conductive layer is an electroless plating copper film, the etchant of the present invention not only removes the base conductive layer and roughens the circuit surfaces, but also can dissolve and remove the palladium catalyst for electroless plating which is present on the insulating material surface in the spaces between the circuits, whereby copper migration between the circuits can be prevented.
In the above semi-additive method, the electrolysis copper plating layer is occasionally covered with an etching resist when removing the base conductive layer to prevent dissolution of the copper electroplating layer which becomes a circuit pattern. In this instance, the aforementioned tin, tin lead alloys, nickel, or gold may be used as the etching resist.
As described above, the present invention provides an etchant free from problems such as instability of the liquid composition and unpleasant odor, and exhibiting a high etching rate.
In addition, the present invention provides an etchant exhibiting only very slight corrosion even if a small amount of residue is left on the surface and capable of producing a roughened surface when used for microetching.